That's all up to you. Except for the really low end silicon based TIMs, which stock TIM is not, it's not going to degrade to the point where it makes a noticeable difference in temperatures for at least three years. To quote TomsHardware:
So 3 years is really when you should think about doing it, but again it's going to depend on how your temperatures are looking. As for which TIM to use, I'd recommend Arctic MX-4. It's inexpensive, versatile, and, to post an abbreviated chart from the same TomsHardware article, it's among the best.
The thermal maximum of any Ryzen series processor should be treated as 90*C, though it's actually slightly higher, it's good to leave yourself a little headroom, and you're not going to get anywhere near that unless you use a program designed to use all the threads fairly efficiently. I say that because what's "good" for me may not sound "good" for you, in your opinion, so just treat anything under 80*C as definitely fine, and anything over 90*C as a cause for concern.
